Plasma arc gas heater



Filed Jan. 22, 1968 Nov. 24, 1970 J MlCHAELls 3,543,084

PLASMA ARC GAS HEATER 2 Sheets-Sheet 1 47 GAS FLOWIN GAS FLOW OUTINVENTOR JOHN L. MICHAEL/5 BY flvgw g ATTORNEYJ Nov. 24, 1970 J.MlCHAELlS 3,543,034

PLASMA ARC GAS HEATER Filed Jan. 22, 1968 2 Sheets-Sheet 2 //vv/vr0/?JOHN L. MICHAEL/5 ATTORNEYS United States Patent O US. Cl. 315-111 13Claims ABSTRACT OF THE DISCLOSURE A plasma arc heater having a pluralityof hollow electrodes forming a central axial passage for material beingheated is disclosed. Each electrode is in the form of continuous coiledmetal tubing, having at least three turns. A first turn of largerdiameter is secured in electrically conductive relationship to acylindrical mounting ring, the additional turns being of a smallerdiameter than the first turn, preferably by at least twice the diameterof the tubing itself so that the smaller diameter turns will fit withinthe larger diameter turns. The tubing of each electrode is coiled, forexample, generally in the form of a helix or spiral, the smaller turnsbeing within the larger turn and successive smaller turns extendingaxially beyond said mounting ring. The mounting ring has openingsthrough which the ends of the tubing pass for introducing andwithdrawing a cooling fluid. The complete arc heater assembly is a stackof essentially duplicate electrode subassemblies, adjacent subassembliesbeing separated by an electrically insulating ring. The insulating ringis of a larger inside diameter than the mounting ring, thereby providinga space between the outside of the smaller coils and the interiorsurface of the insulating ring, and shielding the in sulation. Alsoprovided are means for reversing the polarity of adjacent electrodes inthe heater by external circuits to cause two electrodes to alternatelyperform the same function, e.g., as the cathode for an arc. An arcrotating coil surrounds the heater and is connected electrically inseries with the electrodes.

BACKGROUND OF THE INVENTION The present invention relates to a plasmaarc heater, particularly for gases.

A well known design for such heaters comprises a series of hollowelectrodes forming an axial passage for the material being heated andhaving arc rotating means, such as an electromagnet, which applies atangential component of force to the arc and causes the arc to rotate.Commonly the hollow electrodes are cooled by a fluid such as water withsuitable insulating means being provided between the source of coolingwater and the apparatus. As is well known, de-ionized water is a poorconductor of electricity and a suitable length of plastic tubing betweenthe heater and the source of cool water provides insulation. Previouslyproposed cooling means involve expensive machined or cast hollowelectrode elements, having relatively thick walls to contain the Waterflow at high velocity at high pressure. In addition to being expensiveand diflicult to fabricate, the thickness of the walls is a seriousfactor in providing adequate cooling. Since the input of energy and theconsequent heating of the gas passing through the apparatus depends uponthe cooling capability and efiiciency of the cooling apparatus, theimportance of the efficiency of the cooling means will become apparent.

Another difliculty encountered is related to the fact that the cathodedeteriorates at a rate many times that of the anode, requiring morefrequent replacement of the cathode.

SUMMARY OF THE INVENTION One of the objects of the present invention isto over- 3,543,084 Patented Nov. 24, 1970 come the foregoing and otherdifliculties and problems encountered in the art.

Another object of the invention is to provide electrodes of simplemanufacture and of an economical material which accomplishes theforegoing object.

Another object of the invention is to provide means for controllingerosion of the electrodes, and particularly the cathode.

Briefly, the present invention is directed to a plasma arc heater havinga plurality of hollow electrodes which are aligned to form an axialpassage for the material being heated. The electrodes each consist ofcontinuous coiled metal tubing of substantially uniform diameter, thecoil having at least three turns, including a first turn of largerdiameter. The tubing for this turn is secured within a cylindricalmounting ring and is in electrically conductive relationship therewith.The additional turns of tubing are of a diameter smaller than the firstturn and thus are closer to the axis of the heater. These additionalturns are generally helical, are located within the large turn, andextend axially of the heater so as to provide an axial passage. Themounting ring has two openings through which the tubing passes forintroducing and withdrawing a cool ing fluid. The axial length of eachelectrode is greater than the axial length of its corresponding mountingring, by reason of the helical arrangement of the smaller windings. Whenthe heater is assembled, adjacent electrode subassemblies are,therefore, spaced apart by suitable spacer rings and by cylindricalinsulating rings which also serve to electrically insulate adjacentelectrodes. These insulating rings have a greater inner diameter thanthe exterior diameter of the electrode turns, whereby when the archeater is assembled and the elements are axially aligned, a space willremain between the electrode and the insulating cylindrical ring. Thisspace protects the insulation from the arc, and may also be used toprovide further cooling of the heater by including therein suitablecooling coils adapted to carry a cooling fluid. The heater electrodesare interconnected so that a pair of adjacent electrodes may serve asalternate cathodes for a single anodic electrode, thereby reducing thedeterioration of the heater cathode. This is accomplished by providing avoltage on first one and then the other of the two cathodes which willserve to draw the are from the anode first to one cathode and then tothe other.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and additional objects,features and advantages of the present invention will become apparent tothose skilled in the art from a consideration of the followingdescription of a preferred embodiment thereof, taken with theaccompanying drawings, in which:

FIG. 1, in cross-section, illustrates an embodiment of the inventioninvolving the use of subassemblies of mounting rings and coiled metaltubes to provide a series of hollow electrodes providing a passage forthe arc and for the gas being heated.

FIG. '2 in perspective, and partially in section, illustrates oneembodiment of the coiled metal tube electrode of FIG. 1.

FIG. 3 illustrates a suitable embodiment of a supporting frame ormounting ring having passages through which the coiled metal tube leadsto introduce and withdraw cooling fluid.

FIG. 4 shows a suitable electrical circuit for providing a change inpolarity of a given electrode.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring more specifically to theapparatus illustrated in FIGS. 1-3, the apparatus comprises electrodes1a, 1b, and 10, each of which comprises a continuous metal tube, as ofcopper for example, coiled to form the electrode. Each coil has a turn50 of larger diameter than the turns, with the larger coil being within,and secured in electrically conductive relationship to a cylindricalmounting ring 2, preferably constructed of a conductive material. Oneend of the larger turn, as is shown more particularly in FIG. 2, passesthrough an opening 6 in the mounting ring for connection to a source ofcoolant. The other end of the coil leads through a passage 7 in themounting ring also for connection to the coolant source. These passages6, 7, are more particularly illustrated in FIG. 3. Each coiled tubeelectrode comprises a tube of uniform diameter, the coil having, inaddition to turn 50, at least two turns 51, 52, 53, having a generallyspiral or helical form coaxial with its corresponding mounting ring, butof a diameter smaller than the first turn 50. Preferably the helicalturns 51, 52, 53 are smaller than turn 50 by an amount at least equal totwice the diameter of the tubing whereby, turn 51, as shown in FIG. 1,fits snugly within the larger turn 50, and turns 52 and 53 form a helixextending axially along the heater. Each mounting ring and itscorresponding coiled electrode forms a subassembly, and a plurality ofthese subassemblies may be secured together to form the plasma archeater of the present invention. In the embodiment shown the mountingrings 2 are assembled with adjacent rings separated by cylindricalinsulating rings 11. These insulating rings are coaxial with themounting ring and, as shown, have a larger interior diameter than saidmounting rings. The fact that the smaller turns 51, 52, 53, of thecoiled electrode are of the dimension specified provides a space betweenthe electrode and the insulating ring. As is shown also, the fact thatthe smaller turns of the electrode coil are of the specified dimensionarrangement causes the arc to jump between smaller or inner adjacentcoils 52, by providing the proper spacing.

In the embodiment illustrated in FIG. 1 cylindrical rings 12 areinterposed between the insulating rings 11 and the next adjacentsupporting frames or mounting rings 2. These spacer rings 12, as shownare of greater internal diameter than both the mounting rings 2 and theinsulating rings 11. Another continuous cooling coil 8 is securedinteriorly of and adjacent the interior surface of each spacer ring 12,with openings 9, 10, being provided in the spacer ring 12 for theingress and egress of a cooling fluid. Alternatively, the spacer ringcooling coil may be located exteriorly of the rings. It will thus beseen that when the subassemblies of electrodes and cooling coils arestacked with the insulating and spacing rings to form an arc heater, thesmaller turns of the electrode coils 1a, 1b, and 10 will be radiallyspaced from the spacer ring 12 and the spacer ring cooling coils 8. Inthis way the smaller turns of the electrode coils shield the insulatingrings 11 and, to a lesser extent, shield the spacer rings and theirassociated cooling coils from the heat of the arc. Since the coolingcoils are of tubular metals such as copper, silver, or nickel and may beeasily fabricated on a mandrel, the economies will be apparent. Thecooling coil tubing is from about 0.25 to 0.75 inch, outside diameter,and may have a thin wall, for example 0.028 inch to 0.1 inch inthickness. Also, the electrodes are fabricated of standard tubing, whichis available with various strengths and alloy compositions, and, ifdesired, may have a bi-metallic type of construction. It is preferredthat the circulation of the water be in the turbulent range, for examplelinear feet per second or greater, to assure maximum heat transfer andto assure no dead spots in any portion of the tubes. Further, turbulentflow assures a high heat transfer rate. In assembling the tubing it maybe coiled on a mandrel and then dipped in silver solder or a brazingalloy for the particular metal type of the tube. Although the physicalbonding of the tubular element is not essential, it is preferred thatthis be done.

The plasma arc heater shown in FIG. 1 includes a coil 45 encircling thegenerally cylindrical plasma heater. This coil is connected electricallyin series with the power supply to the heater to create a magnetic fieldwithin the are heater. The coil comprises a continuous wire 46which atone end is connected to a suitable source of electricity, shown forillustrative purposes as direct current batteries 47, 48. The batteriesor other source of direct current are arranged to apply the propercurrent and voltage to the heater, with the internal impedance thereofbeing sufficient to provide a high degree of regulation. The negativeterminals of the sources 47, 48, of electricity are connected to one endof the continuous wire 46 forming the coil, while the positive terminalof one of the sources 48 is connected by a conductor 44 to the lowerelectrode 1c. The positive terminal of the other source 47 ofelectricity is connected by a conductor 43 to the upper electrodeassembly 1a. The other end of the continuous wire 46 forming the coil isconnected by another conductor 49 to the intermediate electrodesubassembly 1b. Accordingly, the top and bottom electrodes will bepositive and the intermediate electrode will be negative thus providingfor an are from one electrode to the next. The coil 45 includes suitableelectrical insulation 50, surrounding the continuous wire 46 and themagnetic field it produces within the heater exerts a generallytangential force on the are which causes the arc to continuously move,or rotate, around the circumference of the electrodes. If desired morethan one electromagnetic coil may be utilized. For purposes of claritythe complete structure of the gas heater is not shown in FIG. 1. Theapparatus includes means for feeding the stream of gas being treated,with the direction of flow being indicated in the drawing by arrows.Further, the apparatus may include a plurality of inlets for gas orseveral types of gas, preferably in the form of tangential conduitswhich provide a swirl to the gas. These gas feeding means may be at theinlet end as well as spaced along the length of the gas heater toprovide for feeding cool and/ or hot gases as may be desired.

FIG. 4 illustrates means whereby the polarity of a given hollowelectrode may be reversed to minimize wear. With the rotating arc,erosion is greatly minimized, as has been known for nearly 50 years.Such rotation, however, does not solve the problem since an arc inrotating, moves in a series of pinpoint steps around the circumferenceof the electrode. For example, as has been shown by experiments, arcrotation may be in the neighborhood of 70,000 revolutions per minute insome specific designs. If it is assumed that the arc changes positionaround the circumference three times to make one revolution, at thegiven rate of rotation this is 210,000 pinpoint contacts per minute or3,500 pinpoint contacts per second on a given electrode. Accordingly,the circuit of FIG. 4 illustrates a new and improved technique toalternately use two mechanically separate electrodes, and by externalelectrical circuits cause them each to function alternately as anelectrode of a given polarity. The electrodes 1b and 1c are suchmechanically separate electrodes, and may be the correspondingelectrodes of FIG. 1. A suitable source of direct current potential isapplied across the conductors 35, 41, one of which is connected to anupper electrode such as subassembly 1a shown diagrammatically. The otherconductor is connected to a center tap of a coil 32, the terminals ofwhich are connected across a capacitor 33 to form a conventional LCtuned circuit 34. Two conductors lead from suitable intermediate taps onthe coil 32 to diodes 30 and 31, respectively, which in turn areconnected by means of conductors 37 and 39, respectively, tocorresponding electrode assemblies 1b and 1c. When the tuned circuit 34oscillates, an alternating current of a suitable voltage isproduced'which, when fed through diodes 30 and 31, provides periodicreversal of the polarity of electrodes 1b and 1c with respect to eachother, first one and then the other becoming more negative. An arcoriginating at electrode 1a, as anode, will, therefore, strikeelectrodes 1b and 1c alternately as one becomes more negative than theother, and thus these electrodes will alternately operate as cathodes.

The oscillations in the tuned LC circuit 34 may be excited by, forexample, a source of alternating current energy. An AC generator 60which may operate at approximately 1750 cycles per second may be asuitable source of such energy. The resulting A.C. energy in the coil 32of the tuned circuit cooperating with rectifier diodes 30 and 31alternately adds to the main power supply and causes the arc to strikecathode 1b and 1c alternately at a rate of 3,500 separate pinpointcontacts per second. The alternating use of these electrodes 1b and 10as the cathode for the are from anode 1a enormously prolongs the usefulservice life of this pair of elec trodes. Although an external source ofalternating current such as the generator 60 may not be required in allapplications to provide the reversing current in coil 32, neverthelesssome suitable means for alternating the DC polarity of electrodes 1b and10 with respect to each other is contemplated. Coil 32 and coil 48' aremagnetically wound on one core to serve as a transformer, or coil 32 mayoperate separately as an inductance only.

In each of the embodiments of FIGS. 1 and 4, magnetic field coil 45 maybe supplied with D.C. energy from a power supply separate andindependent of the power supply of the arc, while in the embodiment ofFIG. 4, this coil is supplied from a DC source separate from the excitersource 60.

By the alternate use of the adjacent cathodes 1b, 1c in the arrangementshown in FIG. 4, the are simulta neously rotates by reason of themagnetic field created by coil 45 and in addition alternates between theadjacent cathodes. This provides much more rapid stepping or locationchanges at which the arc strikes. Further, it permits the use of aweaker magnetic field, thus minimizing danger of blowing out the arc,which may occur with a strong magnetic field.

While the preferred embodiment involves the use of a direct current arc,it will be apparent that the invention is also useful with analternating current arc.

Conventional arc starting methods and means, not illustrated, are usefulwith the apparatus of the invention. One common technique is to provideionized gas in the desired arc path prior to starting the are. This maybe done either by introducing pro-ionized gas into the gas heater, or bysoldering fuse wires to the electrodes, which vaporize upon havingcurrent passed therethrough to provide the ionized atmosphere. Anothermethod is to apply high voltage, high frequency pulses across theelectrodes.

Although the invention has been described in considerable detail withreference to certain preferred embodiments thereof, it will beunderstood that variations and modifications can be effected withoutdeparting from the spirit and scope of the invention as describedhereinabove and as defined in the appended claims.

I claim:

1. A plasma arc heater comprising a plurality of axially aligned hollowelectrode subassemblies forming an axial passage for material to beheated, each electrode subassembly including an electrode formed fromcontinuous metal tubing coiled in at least three turns and a conductingcylindrical mounting ring, each electrode including a first turn securedwithin and electrically connected to its corresponding mounting ring,each of said electrodes further including second and third turns forminga helix within said first turn to provide said axial passage, said helixhaving a greater axial extent than said mounting ring; an electricallyinsulating ring separating adjacent ones of said plurality of electrodesubassemblies; and circuit means including a DC source of power forcausing a DC are to flow between the helical turns of said electrodes,

said helical turns serving to shield said insulating rings from saidarc.

2. The arc heater of claim 1, wherein said metal tubing for each saidelectrode is of substantially uniform diameter.

3. The arc heater of claim 1, wherein said second and third turns are ofa diameter smaller than the diameter of said first turn, whereby saidsecond and third turns are closer to the axis of said heater than saidfirst turn.

4. The are heater of claim 1, wherein the ends of the coiled electrodetubing for each said subassembly extends through its correspondingmounting ring and is adapted to receive cooling fluid.

5. The arc heater om claim 1, wherein the internal di ameter of saidinsulating rings is greater than the diameter of said helical turns,whereby said turns are spaced inwardly from said insulating rings.

6. The arc heater of claim 1, further including arc rotating meansconnected to said DC source of power.

7. The are heater of claim 1, wherein said heater includes first, secondand third electrode subassemblies in sequence, said first electrodeconstituting an anode for said arc, and said second and third electrodesconstituting adjacent, mechanically separate cathodes for said arc, saidcircuit means varying the DC voltage on said cathode electrodes wherebysaid are will flow from said anode to alternate ones of said cathodes.

8. The arc heater of claim 7, wherein said circuit means includes asource of alternating current for applying a varying voltage to saidcathodes.

9. The arc heater of claim 7, wherein said circuit includes tunedcircuit means connected across said second and third electrodes.

10. The are heater of claim 7, further including spacer rings betweeneach said insulating ring and one of the adjacent mounting rings, theinternal diameter of said spacer rings and of said insulating ringsbeing greater than the diameter of said helical turns, whereby saidturns are spaced inwardly from said insulating rings.

11. The are heater of claim 10, further including cooling coils,adjacent said spacer rings.

12. The are heater of claim 11, wherein the ends of the coiled electrodetubing for each said subassembly extends through its correspondingmounting ring and is adapted to receive cooling fluid.

13. In a method of operating a plasma arc heater comprising a directcurrent source and at least a first, second and third generallycylindrical electrodes axially spaced in sequence, the improvement ofapplying a DC voltage to said first electrode to form an anode, applyinga DC voltage to said second and third electrodes to form a cathode, andvarying said voltage on said cathode electrodes whereby an arc flowingfrom said anode to said cathode will strike said cathode electrodesalternately.

References Cited UNITED STATES PATENTS 2,964,679 12/ 1960 Schneider etal. 315-111 3,140,421 7/1964 Spongberg 315111 3,360,682 12/1967 Moore315-111 3,402,366 9/ 1968 Williams 331-94.5 3,43 6,679 4/1969 Fenner331--94.5

RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R.

